1. Field of the Invention
The present invention relates to a density processing method in an image forming apparatus such as a copying machine.
2. Description of the Prior Art
In a digital color copying machine, a document is first irradiated by an exposure lamp in a scanner section, and its reflected light is detected by a CCD (Charge Coupled Device) and sequentially converted into an electric signal. In this case, an image of the document is separated for each color and for each pixel by the CCD into an electric signal corresponding to the density of each pixel. This electric signal is sent to an image processing section.
In the image processing section, an output of the CCD is digitally converted and then, the variations of the CCD, the exposure lamp and the like are corrected for each signal having colors (B (blue), G (green) and R (red)) (BGR signal) in a shading correcting section. The BGR signal is converted into a toner density signal (a YMC (yellow, magenta and cyan) signal) in a BGR-YMC converting section. In addition, a BK (black) signal is produced from the YMC signal in a BK producing section.
Thereafter, in the YMC signal and the BK signal, the density levels of the respective colors are corrected in accordance with the characteristics of a filter and toner in a color correcting section. Furthermore, in a color converting section, processing, such as color conversion of a designated color, trimming or masking, is performed. Thereafter, in a density processing section, the level of a digital density signal sent is converted depending on a developing color, the copy density designated in an operating section, the type of document image designated in the operating section, and the like. Thereafter, the digital density signal is sent to a printing section through a variable magnification and movement processing section for performing variable magnification and movement processing in the main scanning direction of an image, to accomplish recording on paper.
Meanwhile, in the digital copying machine or the digital color copying machine, density processing using a dither method has been generally performed in the density processing section so as to obtain a gray scale image. A digital color copying machine already developed by the applicant of the present application is so adapted that data representing a gray level of a recording pixel relative to a gray level of a reading pixel (hereinafter referred to as input gray level-output gray level data) have been previously generated using a dither matrix having 2.times.2 pixels as one block and stored in a memory device, to find data representing a gray level of a recording pixel (hereinafter referred to as output gray level data) corresponding to data representing a gray level of a reading pixel (hereinafter referred to as input gray level data) inputted on the basis of the input gray level-output gray level data. There are 256 gray levels of the reading pixel, and there are 64 gray levels of the recording pixel. A density processing circuit in this copying machine is shown in FIG. 1.
The density processing circuit comprises a CPU 10, a data ROM 11, a table memory 13, and an address generating circuit 12. A plurality of types of input gray level-output gray level data previously generated are stored in the data ROM 11 depending on a developing color (M, C, Y or BK), the copy density designated in an operating section, and the type of document image designated in the operating section.
The CPU 10 transfers one type of input gray level-output gray level data corresponding to the developing color, the copy density designated in the operating section, and the type of document image designated in the operating section out of the plurality of types of input gray level-output gray level data from the data ROM 11 to the table memory 13. Image data representing a gray level of reading pixel (input gray level data) and a signal representing the position of the reading pixel (a pixel position signal) comprising a line signal HSYNC and a dot signal CLK (not shown) are sent to the address generating circuit 12. The address generating circuit 12 outputs a signal for specifying an address storing output gray level data corresponding to the input gray level data and the pixel position signal sent (a pixel in the dither matrix which corresponds to the reading pixel) out of addresses in the table memory 13. Consequently, the output gray level data stored at the specified address is outputted from the table memory 13.
In this copying machine, the number of gray levels of the reading pixel is 256, and the number of pixels constituting one block in the dither matrix is 4. Accordingly, the number of one type of input gray level-output gray level data is 256.times.4=1024. Such input gray level-output gray level data differ for each developing color (M, C, Y or BK), for each copy density designated in the operating section, and for each type of document image designated in the operating section. It is assumed that there are four types of developing colors, there are 15 levels of the copy density designated in the operating section, and there are three types of document images designated in the operating section, that is, a character, a photograph and a character-photograph mixture. In this case, if the data ROM 11 is a 8-bit memory, 1024.times.4.times.15.times.3=184320 bytes is required as the capacity of the data ROM 11. In such a copying machine, therefore, a large-capacity memory device is required to store the input gray level-output gray level data.
Therefore, the applicant of the present application has proposed a method of generating input gray level-output gray level data for each developing color (M, C, Y or BK) and for each type of document image designated in the operating section and making the adjustment of an output gray level based on the copy density designated in the operating section on the basis of the input gray level-output gray level data generated so as to reduce the capacity of the data ROM 11. Specifically, it is a method of finding an output gray level relative to an input gray level on the basis of input gray level-output gray level data corresponding to a developing color and the type of document image designated in the operating section and then, increasing or decreasing the output gray level found by a gray level corresponding to the copy density designated in the operating section, to obtain an output gray level. That is, it is a method of merely adding or subtracting an offset value to or from the output gray level found by the input gray level-output gray level data.
FIG. 19 shows input gray level-output gray level data corresponding to a developing color and the type of document image designated in the operating section, and FIG. 20 shows input gray level-output gray level data obtained by so correcting the input gray level-output gray level data shown in FIG. 19 that output gray level data is larger by a predetermined gray level by the above described method.
It is preferable that the relationship between input gray level data and the density at which printing is actually done is linear. However, the relationship between output gray level data and the density at which printing is actually done is not linear due to characteristics which are inherent to the copying machine. Accordingly, if the relationship between the input gray level data and the output gray level data is made linear, the relationship between the input gray level data and the density at which printing is actually done is not linear. Therefore, input gray level-output gray level data is so generated that the relationship between input gray level data and the density at which printing is actually done is linear in consideration of the relationship between output gray level data and the density at which printing is actually done which is inherent to the copying machine. Therefore, the characteristics of the input gray level-output gray level data generated become non-linear characteristics and characteristics which are inherent to output gray levels.
In a method of merely adding or subtracting an offset value to or from an output gray level found by input gray level-output gray level data, if input gray level-output gray level characteristics obtained by the method shown in FIG. 20 are compared with the original input gray level-output gray level characteristics shown in FIG. 19, the value of an output gray level at a point on the characteristic curve is changed from the value of an output gray level at a corresponding point on the original characteristic curve. Accordingly, the relationship between the output gray level data and the density at which printing is actually done in the original input gray level-output gray level characteristics cannot be maintained. Therefore, the relationship between the input gray level data and the density at which printing is actually done is not linear. Further, in the method of merely adding or subtracting an offset value to or from an output gray level found by input gray level-output gray level data, the range of output gray levels is extremely decreased.